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Vintage Tape (Audio), Cassette, Wire and Magnetic Disc Recorders and Players Open-reel tape recorders, cassette recorders, 8-track players etc.

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Old 27th Jul 2020, 11:01 am   #1
DMcMahon
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Default Strange Line on Back of Tape

I have just borrowed my friends Uher 4000 Remote Monitor R2R to do some comparisons with my 4000, to hopefully help work out why my 4000 struggles with RWD & FFD (covered in an older Thread).

I see that his tape has some strange lines on the back side of the tape, which is 5" Agfa PEM 368 Back Coated tape (same tape type as my recent 7" tapes that had SSS).

The lines continue along most of the length of tape but there are sections with no lines, sometimes the line is a single fairly clean line like Photo 1, other times it is like Photo 2.

I assume this must be where the thin backside coating has been worn away by something. It is not known if the tape arrived like this (he brought it fairly recently) or whether his 4000 has done the damage.

Looking at his 4000 briefly, there is quite a lot of fine dark dust on the top plate and around tape path area, I will look more closely to see if anything obvious that might be rubbing the backside of the tape.

From a very quick check the tape does not appear to be sticky, in terms of sticky deposits on heads, tape guides etc.

Has anybody seen similar before, any ideas please ?

David
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Old 27th Jul 2020, 12:03 pm   #2
G6ONEDave
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Default Re: Strange Line on Back of Tape

Not had that particular issue but would be looking carefully at the felt pressure pads. Maybe they have gone hard or got some foreign matter in them. Also the capstan shaft for excessive wear.
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Old 27th Jul 2020, 12:55 pm   #3
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Default Re: Strange Line on Back of Tape

I reckon it's deliberate to mark the back of a matt finish tape.
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Old 27th Jul 2020, 1:24 pm   #4
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Default Re: Strange Line on Back of Tape

Quote:
Originally Posted by merlinmaxwell View Post
I reckon it's deliberate to mark the back of a matt finish tape.
My friend has now confirmed that he brought a batch of these Agfa tapes and this is the only one with the line marks and the only one of the batch that he has used a lot on the 4000.
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Old 27th Jul 2020, 1:32 pm   #5
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Default Re: Strange Line on Back of Tape

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Originally Posted by G6ONEDave View Post
Not had that particular issue but would be looking carefully at the felt pressure pads. Maybe they have gone hard or got some foreign matter in them. Also the capstan shaft for excessive wear.
No pressure pads of any sort on the 4000.

From a quick inspection the Capstan shaft (large diameter) has some normal wear, but not excessive.

Looks like only 2 items are making contact with the back of the tape:-

1. Of course the Pinch Wheel, looks in OK condition.

2. The vertical side edges of the metal hum flap that is pushed towards the PB head in Play mode. Need to do some tests to conclusively prove but now am strongly thinking that the hum flap is the most likely culprit. If so then this means that machines with this type of hum flap may not be compatible with back coated tape usage.

David
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Old 27th Jul 2020, 6:06 pm   #6
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Default Re: Strange Line on Back of Tape

I would not expect any tape deck to press a metal flap of any type onto the reverse of the tape, without having a thin felt pad in place. The bare metal flap would be harder than the tape medium and without some softer material between it and the tape will cause tape damage. The original felt pad has probably worn away or dropped off and got lost years ago. This 'hum flap' as far as I know is to offer some slight pressure from behind the tape, in order to keep the tape in contact with the heads.
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Old 27th Jul 2020, 7:02 pm   #7
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Default Re: Strange Line on Back of Tape

The main function of the Hum Flap is to shield the Playback head from electrical interference such as hum during Playback.

I have never seen a pad in any Hum Flap across many machines, this maybe is because the metal flap is not supposed to contact the tape itself.

I have found a third contact point on the back of the tape. There is a black plastic block that the little copper sprung arm (that pushes against the Hum Flap) is mounted to.

There are 2 raised vertical curved ridges on the block roughly inline with the Record head, when Play is selected, the ridges push the tape forward, so these ridges will be making contact with the back of the tape when Playing. As the ridges are plastic and curved would think that normally they should not damage the back coating.

By manually pushing forward all the way, the metal Hum Flap I can see that both sides of its vertical edges make contact with the tape and maybe push the tape forward a little bit. These edges are angled out at 45 degrees so the tape contact is maybe across a sharp corner edge at that time. However when Play is selected the Pinch Wheel pushes the tape forward onto the capstan shaft and possibly moves the tape enough to move it away forward of the Hum Flap right edge. Similarly the plastic ridges may also move the tape forward enough to clear the left hand Hum Flap edge, it is nearly impossible to actually see, so it is possible that during Playback that the tape does not make contact with the Hum Flap.

A test I am going to try is to find a short section of the tape that has no line on the back then run that section through in Play one or more times to see if a line starts to be made on the back of the tape. If line can be created then repeat the test with the Hum Flap removed. If no line with Hum Flap removed then that would suggest the the Hum Flap is contacting the tape and rubbing off some of the back coating. If line still being created with no Hum Flap then that could suggest that the plastic ridges are making the line.

We will see later.

David
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Old 28th Jul 2020, 8:25 am   #8
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Default Re: Strange Line on Back of Tape

Sounds like a good way to try and prove the fault. If there is a sharp area in contact with the tape, then that will be the problem. It only takes a few thou of an inch to cause damage and a good magnifier or mobile phone camera will be useful to get a closer look at possible problem areas.
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Old 28th Jul 2020, 3:19 pm   #9
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Default Re: Strange Line on Back of Tape

Photo 1 shows the area (not in Play mode), it can be seen that the Hum Flap vertical sides are a little behind the black ridges on the left side of the Hum Flap.

Photo 2 in Play mode with no tape, black ridges in front of the Hum Flap edges.

Photos 3 and 4 in Play mode with tape, once again black ridges in front of the Hum Flap edges, however the tape after the ridges moves back and looks very close to the Hum Flap left edge (cannot really see right edge).

Even with large magnifier and lamp, extremely difficult to really see if the tape fouls the hum Flap edges, looks very close though.
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Old 28th Jul 2020, 3:29 pm   #10
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Default Re: Strange Line on Back of Tape

If you have access to a dentist mirror, you might be able to view the 2 black points that contact with the tape. I would try a dentist mirror with a small light souce and with the third hand (friendly assistant) camera zoom on a mobile phone to enlarge the image. Otherwise it looks like a strip down, unless the parts in question can be got out for close inspection.
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Old 28th Jul 2020, 11:35 pm   #11
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Default Re: Strange Line on Back of Tape

Someone may have misthreaded the tape and had it scraping against the head cover or somewhere in use.Seen this before.
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Old 29th Jul 2020, 9:38 pm   #12
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Default Re: Strange Line on Back of Tape

Tried small dentist mirror but impossible to see anything due to no suitable access point.
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Old 30th Jul 2020, 9:55 pm   #13
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Default Re: Strange Line on Back of Tape

Did numerous repeat cycles of Play, RWD & FFD but could not create a line/mark on the back coating of the tape.

Hard to know where the original damage to the tape occurred, there was plenty of dark dust inside his 4000 but maybe the tape was already damaged when he got it and subsequently some of the back coating came away or as Ben suggests maybe it was badly threaded somehow and abraded on something ? it is definitely OK at the moment.
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